V4L/DVB (13619): v4l2-spec: Adds EBUSY error code for S_STD and QUERYSTD ioctls

During review of Video Timing API documentation, Hans Verkuil had a comment
on adding EBUSY error code for VIDIOC_S_STD and VIDIOC_QUERYSTD ioctls. This
patch updates the document for this.

Signed-off-by: Muralidharan Karicheri <m-karicheri2@ti.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>